The power-up of chips that have several power supply sources is a difficult problem. Indeed, to work correctly any chip needs a minimum supply voltage. When there are several power supply sources with different voltage on each, it is necessary to determine which of these sources can provide a high enough voltage to the chip. Once this decision is taken, a mix of these valid power supply sources can be done to supply the chip. The decision or selection circuit is difficult to design because the decision circuit itself is also not supplied by one constant power supply source but by several and it has to work whatever the values of the voltages on these sources are.
As known from the prior art FIG. 1 shows a solution for a decision circuit which is used to generate a power supply from two power supply sources VDD1 and VDD2. It is based on a feedback loop.
The circuit comprises a comparator CMP with a comparator input 1 connected to a first voltage input IN1 and a comparator input 2 connected to a second voltage input IN2. The first comparator input 1 is applied with the first supply voltage VDD1 while the second comparator input 2 is applied with the second supply voltage VDD2. A first diode D1, which is connected in parallel to a first controllable switch SW1, is connected on the one side to the first voltage input IN1 and on the other side to a circuit output OVDD. A second diode D2, which is connected in parallel to a second controllable switch SW2, is connected on the one side to the second voltage input IN2 and on the other side to the circuit output OVDD. The control input of the first controllable switch is controlled by the output 3 of the comparator CMP. The output 3 of the comparator CMP is also connected to an inverter INV, whereby the output 6 of the inverter INV controls the control input of the second controllable switch SW2. Both power supply connectors 4 and 7 of the comparator CMP and the inverter INV are connected to the output voltage VDD.
The first switch SW1 is conducting, if the first control voltage SWVDD1 is lower than the difference between VDD1−Vt or if the first control voltage SWVDD1 is lower than the difference between VDD−Vt. In this case the first voltage VDD is VDD1. If the first control voltage SWVDD1 is higher than the difference between VDD1−Vt and higher than the difference between VDD−Vt, the first switch SW1 is not conducting and the voltages VDD and VDD1 are independent. The voltage Vt is a constant voltage. In principle this is also valid for the second switch SW2. The second switch SW2 is conducting, if the second control voltage SWVDD2 is lower than the difference between VDD2−Vt or if the second control voltage SWVDD2 is lower than the difference between VDD−Vt. In this case is the output voltage VDD is VDD2. If the second control voltage SWVDD2 is higher than the difference between VDD2−Vt and higher than the difference between VDD−Vt, the second switch SW2 is not conducting and the voltages VDD and VDD2 are independent.
At power-up, the diodes D1 and D2 pull up the node OVDD, which is the output of the circuit, to the voltage VDD equal to the maximum of the three values VDD1−Vdiode, VDD2−Vdiode or O. The voltage Vdiode is the diode voltage and is equal for both diodes D1 and D2.
If this voltage VDD is lower than the constant voltage Vt, the comparator output 3 and the inverter output 6 will be floating and therefore be undefined. In some cases, this can lead to a large short circuit current. Let's assume for example that the first supply voltage VDD1=Vt+Vdiode, the second supply voltage VDD2=0, the first control voltage, also called comparator output voltage, SWVDD1=0 and also the second control voltage, also called inverter output voltage, SWVDD2=0. The first switch SW1 is conducting and will try to force the voltage VDD=VDD1=Vt+Vdiode on the node OVDD. But because the control voltage SWDVDD2=0 and this is lower than VDD−Vt=VDD1−Vt=Vt 30 Vdiode−Vt, the second switch SW2 is also conducting and will try to force an output voltage VDD=0 on the node OVDD.
In the following, the voltage difference VDD1−Vdiode or the voltage difference VDD2−Vdiode is supposed to be higher than the constant voltage Vt. The stable state of the circuit will be determined. The comparator output 3 will be 0 if the first voltage VDD1 is higher than the second voltage VDD2 and the comparator output 3 will be the maximum voltage if the second voltage VDD2 is higher than the first voltage VDD1. The maximum voltage is the higher value of the two values VDD1−Vdiode or VDD2−Vdiode. The inverter INV, also supplied with the output voltage VDD, will produce a second control voltage SWVDD2, which is the maximum of VDD1−Vdiode and VDD2−Vdiode, if the first control voltage SWVDD1=0. The inverter INV will produce a second control voltage SWVDD2=0 if the first control voltage SWVDD1 is the maximum of VDD1−Vdiode and VDD2−Vdiode. The first control voltage SWVDD1 switches on the first switch SW1, this means the first switch SW1 is conducting, and the second control voltage SWVDD2 switches off the second switch SW2, this means the second switch SW2 is not conducting, when the first voltage VDD1 is higher than the second voltage VDD2. The control voltages SWVDD1 and SWVDD2 switch off the first switch SW1 and switch on the second switch SW2 when the second voltage VDD2 is higher than the first voltage VDD1. The output voltage VDD is then going from max (VDD1, VDD2)−Vdiode to max (VDD1, VDD2). Similarly, the control voltages SWVDD1 and SWVDD2 will either stay on 0 or go up to max (VDD1, VDD2).
This circuit has several drawbacks. It will not function, if the first voltage VDD1 and second voltage VDD2 are lower than Vt+Vdiode. If the first voltage VDD1 and the second voltage VDD2 are lower than Vt+Vdiode, there can be a high short circuit parasitic current.